Why is the current so large when the three-phase four-wire power distribution system is connected like this?

乱世煮酒论天下

Why is the current so large when the three-phase four-wire power distribution system is connected like this? [Copy link]

 

In the three-phase four-wire power distribution system, three live wires are connected to the motor. The motor is 55kW380V connected in a triangle. A single-phase power supply is connected from the A-phase power supply and the neutral line to power the single-phase equipment such as air conditioners in the equipment. When this single-phase power supply is powered on, the current is very large and can reach hundreds of amperes. However, the three wires connected to the motor end can be removed. In this way, when the single-phase power supply is powered on again, the current will only be a dozen amperes. What is the reason for this?

I did a simple simulation, I don't know if it's correct as shown in the following figure: the current does not change

maychang

[I did a simple simulation, I don’t know if it’s correct]

Of course not!

The six components in the blue box have been short-circuited by the green wire. These six components are equivalent to a wire in the circuit. Can you see it?

乱世煮酒论天下

maychang posted on 2024-9-13 15:16 [I did a simple simulation, I don’t know if it’s right] Of course not! The six components in the blue box have been short-circuited by the green line. These six...

This frame is intended to simulate the winding of an asynchronous motor. Originally, the three-phase motor and the single-phase motor are connected in parallel with phase A. The parameter settings here may not be correct, but the current does not change. It may also be that the transient current is very large, and the ammeter only measures the steady-state current.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-9-13 16:01 This frame is intended to simulate the winding of an asynchronous motor. Originally, the three-phase motor and the single-phase motor are connected in parallel with phase A. Maybe the parameters here are not set correctly, but...

[This frame is intended to simulate the winding of an asynchronous motor. Originally, the three-phase motor and the single-phase motor have the A phase connected in parallel.]

A three-phase motor must be connected to phases A, B, and C, and will not be connected to the neutral wire. In other words, it has nothing to do with the neutral wire. There is absolutely no such thing as [a three-phase motor and a single-phase motor are connected in parallel with phase A].

乱世煮酒论天下

maychang published on 2024-9-13 16:04 [This frame is to simulate the winding of the asynchronous motor. Originally, the three-phase motor and the single-phase motor are connected in parallel with phase A] The three-phase motor must be connected with phases A, B, and C...

What I mean is that three live wires are connected to the three-phase motor U V W, and one live wire and the neutral wire are connected to the single-phase motor. Now only the single-phase motor is powered, that is, only one live wire and the neutral wire form a loop.

led2015

As mentioned on the second floor, the green line is short-circuited here, causing the blue box to be inoperative?

maychang

乱世煮酒论天下发表于2024-9-13 21:35 What I mean is that three live wires are connected to the three-phase motor U V W, and one of the live wires and the neutral wire are connected to the single-phase motor. Now only the single-phase motor is connected...

[I mean three live wires are connected to the three-phase motor U V W, one of the live wires and the neutral wire are connected to the single-phase motor. Now only the single-phase motor is powered, that is, only one live wire and the neutral wire form a loop]

There is no such thing as [three-phase motor and single-phase motor are just A phase connected in parallel].

maychang

乱世煮酒论天下发表于2024-9-13 21:35 What I mean is that three live wires are connected to the three-phase motor U V W, and one of the live wires and the neutral wire are connected to the single-phase motor. Now only the single-phase motor is connected...

Based on the circuit diagram below the first post, it is estimated that [three live wires are connected to the three-phase motor U V W, and one of the live wires and the neutral wire are connected to the single-phase motor] is impossible. Can anyone know how you connected the wires?

maychang

led2015 posted on 2024-9-13 22:54 Did the green line short-circuit here and cause the blue box to be ineffective?

[The second post said that the green line is short-circuited here, causing the blue box to be ineffective?]

According to his circuit diagram, the blue box has no function. But according to the original poster, the three-phase motor is connected to the three-phase AC line, which is not shown in the diagram at all. It should be noted that V1 in the diagram is only one phase, and God knows how he connected it.

qwqwqw2088

It's not the electrician's job to manage electrical work. The key is that they don't understand it.

The line voltage of the three-phase four-wire power distribution system is indeed 380V . Multisim simulation is also used, and the simulation is a 220V circuit.

It is a three-phase imbalance problem.

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This situation should be easy to check. The load is one large and one small, which is easy to distinguish. The small load is a single-phase air conditioner, which is generally equipped with a 4-square wire and cannot withstand a large current of 100 amps. Therefore, when your power supply system is running, use a clamp meter to measure the current of each wire. The current of the single-phase line should be around 10 amps, which is normal. The current of the motor power line is about 110A. The total power line has two phases of 110A and the other is 120A.

If the power supply can operate normally, there is a problem with the measurement.

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If there is no clamp meter to measure the current, you can feel the wire temperature with your hand. If the line is over-current, it will become hot and the circuit breaker will trip.

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In fact, this problem is not a problem from the power supply point of view. After turning on the two loads, just use a meter to measure the small single-phase wire or touch it with your hand. If the current or temperature of the small wire is normal, there is no problem. Single-phase electrical appliances operate normally, and the current generally does not exceed 16A.

Just as the original poster said, the single-phase current has reached several hundred amperes. The single-phase circuit breaker would have tripped long ago and all the fuses would have burned out. The single-phase circuit breaker did not trip, which means that the current is not real.

乱世煮酒论天下

Vibration Test Instrument Published on 2024-9-14 15:53 In fact, this problem is not a problem from the power supply point of view. After the two large and small loads are turned on, just look at the small single-phase wire with a meter or touch it with your hand. The small line...

It is indeed true. The first time I powered on, a 1.5 square wire exploded in less than 3 seconds, burning the insulation and melting the rubber. The second time I replaced it with a 16 square wire and used an oscilloscope to measure the current, which was about 190A. I quickly turned off the switch and removed the wiring to the three terminals of the motor, and the current was only a dozen or so.

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When a single-phase wire burns out, you should check whether there is a short circuit in the line or in the load circuit, rather than increasing the wire cross-section and trying again. If a short circuit occurs when the machine is turned on again, the short circuit fault will be prolonged and cause greater damage.

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When there is no 55KW motor connected to the circuit, the single-phase load starts normally. However, when a three-phase motor is connected, a current of 190A is generated when the single-phase load starts. The source of the large current may be the starting current of the motor.

The starting of a 55KW motor must have a special control and protection circuit, such as a special air switch to control the circuit, with short-circuit and overcurrent protection. When the motor is not working, the air switch is generally disconnected, and the motor has no connection with the three-phase power supply.

There is an AC starter or frequency converter control between the air switch and the motor. When the air switch is closed and no one starts it, the motor will not rotate.

Could it be that your single-phase load starting can start the motor?

zhaoweige

There should be something wrong with the motor control circuit, or the wiring. I'll wait for the answer.

振动试验仪器

Another possibility is that the neutral wire and the ground wire were mixed up during the actual wiring. The motor needed to be connected, but the neutral wire was mistakenly connected. This situation has happened before.

When the motor is a three-phase balanced load, the current in the neutral line is very small, and the best result is zero.

If the current of the neutral line is too high when the motor is running, it means that the three phases of the motor are unbalanced, and the result is that the motor is broken.

振动试验仪器

I just thought of the conditions for this special phenomenon to occur.

There is a problem with the wiring of the three-phase motor. One phase of the power line is not connected. So after the motor is started, the neutral line of the power supply is the working current of the motor. Therefore, as mentioned on the 14th floor, the 1.5 square wire should be the neutral line.

The neutral wire should not be connected to the air switch, so there is a large current of more than 100 amperes on the neutral wire without over-current short-circuit protection.

That is, the workplace is first operated with single-phase power supply, such as turning on the air conditioner and lighting, and then starting the motor. At this time, the starting current of the motor is 4 to 6 times the normal current. Therefore, a super current of 180A briefly appears on the neutral line, and soon the neutral line heats up, smells, and burns out, and the motor makes a rumbling sound and becomes stalled.

振动试验仪器

Judging from the fact that the original poster has not been able to come up with a real answer after all his efforts, the three-phase power supply of the motor should be connected properly. The problem with the three-phase power supply may be that one of the cables is broken, or there is a problem with the output of the controller and a phase is missing, such as a fuse is blown, or a phase is missing when the power is supplied.